A more thorough explanation:

Mapp v. Ohio was a landmark Supreme Court case in 1961. The case was decided 6-3 by the Warren Court. The court held that the Fourth Amendment's protection against unreasonable searches and seizures applied to the states. This meant that unconstitutionally obtained evidence could not be used in state criminal prosecutions.

The case involved Dollree Mapp, whose home in Cleveland, Ohio was forcefully entered by police officers who believed that a suspected bomber was inside the house. While searching her home, officers found pornographic books. Later, Mapp was prosecuted under an Ohio statute for knowing possession of lewd and lascivious material. She was convicted even though the prosecution was unable to produce a valid search warrant.

The majority held that all evidence obtained unconstitutionally, without a search warrant, is inadmissible in state criminal prosecutions. Such evidence was already barred in federal courts, but the majority agreed that the exclusionary rule for unlawfully seized evidence applied to state courts as well, through the Fourteenth Amendment. The Supreme Court insisted that the exclusionary rule had to apply to the states, or else the Fourth Amendment was essentially useless.

This case set an important precedent for protecting citizens' Fourth Amendment rights against unreasonable searches and seizures.